We sent this story to CorteX Racing for fact checking, and the reply we received was too good not to share. Company partner and founder Filip Trojanek went deep, debunking the idea that independent rear suspension is always superior to a solid-axle setup. His response dropped so much knowledge on us we decided to share it here.

We often see solid-axle cars keeping up with or even outrunning their IRS-equipped competitors. Why? The reason is simple: An independent rear suspension doesn’t necessarily perform better than a well-designed solid-axle setup. This is especially true for vehicles with a lot of front weight bias, like a Mustang.

It all comes down to how a car shifts its weight backward when it accelerates. That weight needs to transfer to the rear tires so they can grip the road during power-down, but it takes time to travel through the springs and shocks–especially on a front-heavy vehicle. On a high-horsepower car, that time delay shows up as lost traction.

(IRS does work well on mid- and rear-engine supercars, since their back tires don’t need as much additional weight to put down the power. Even a Corvette gets pretty squirrelly when it has a 52- to 58-percent rear bias.)

The way to speed up the process is to increase the anti-squat geometry. This allows the weight to transfer directly, without waiting for the springs to load up.

However, an IRS with significant anti-squat generally has bad rollsteer characteristics, making this an impractical solution.

The same is not necessarily true for a solid axle. A torque arm design is good for both power-down and cornering because it allows for a relatively high anti-squat value without compromising other geometry. It’s also very tolerant of rear ride-height changes (IRS is extremely sensitive in comparison) due to the stable foreaft instant center location.

If you drive on bumpy surfaces, it’s usually better to reduce anti-squat since it will let the suspension move more, allowing for more tuning and engagement of the shocks and springs. This can be accomplished by changing the angle of the rear lower control arms.

Everyone fixates on track handling, but on the street, what the car has makes an enormous difference in ride comfort due to the sprung-to-unsprung weight ratios. The lighter the car, the more pronounced it becomes and on Lotus Seven type cars (or even Miatas), it's honestly the difference between a comfortable car and one you don't want to drive on an even slightly bumpy road.

I do think it's kind of funny that most of what people do to their cars to make them "handle better" is basically just turning their independent suspensions into more and more like a solid axle. For instance installing super stiff springs or sway bars to try to keep camber from changing through roll.

I have plans for building an fsae car for SCCA autocross in the future and I'm definitely going to be using solid axles (with probably a de Dion in the rear) for sheer simplicity. There are actually tons of benefits to it compared to independent stuff. For instance, it takes far fewer suspension pickup points on the chassis. You also get consistent predictable camber behavior. It also makes a great platform to mount sprung arrow elements to, which is totally legal in that class.

Everyone fixates on track handling, but on the street, what the car has makes an enormous in difference in ride comfort due to the difference in sprung-to-unsprung weight ratios. The lighter the car, the more pronounced it becomes and on Lotus Seven type cars (or even Miatas), it's honestly the difference between a comfortable car and one you don't want to drive on an even slightly bumpy road.

I've put a solid axle in the rear of my V8 MG, which has Miata front suspension and very similar weight and balance to the Miata. I'm having a hard time getting the ride quality in the rear where I want it, and I think it's in large part due to that massive axle bouncing around.

Based on the fun I had loading the Explorer axle for the Ranchero out of the van, I'm pretty sure it's having a substantial impact on the sprung:unsprung ratio... It's not apples to apples comparing it to the A1 VW knuckle on the shelf, but I can't help noticing when I pass them both...

A bit off topic, but loading into the van with the help of an exuberant but cavalier assistant reminded me how much I prefer doing that sort of thing myself. Even if the total time it takes is longer, the number of digits retained per unit time is better.

Solid axles can definitely be a problem in the unsprung weight department, but other than that, they're generally simpler to get setup in a way that works well. Even a solid front axle isn't a bad thing if it's done well.

I prefer IRS for the street. On a smooth track, I am not sure it really matters too much. On the street the suspension has way too much to deal with, potholes, camber changes, surface changes, dirt, ice, snow, rain, and that is before you try to turn

I do think it's kind of funny that most of what people do to their cars to make them "handle better" is basically just turning their independent suspensions into more and more like a solid axle. For instance installing super stiff springs or sway bars to try to keep camber from changing through roll.

In a way, that is actually making them the opposite of a solid axle, because bump on one end will result in droop in the other and vice versa, since the springs are by necessity not located over the contact patches. It is sort of like having a built in Z bar.

If it is not apparent, i consider this to be a virtue and not a vice. Especially when judicious amounts of anti squat are present and the goal is to accelerate as hard as practical over rough terrain.

I’ll tell you this, my ford 8.8 lifts the inside tire on off camber corners on the street. It sucks in the rain or snow, since you can’t whomp on the gas to pull out in traffic.

That's not a solid axle specific problem. It's a "too much weight transfer off the inside rear in turns" problem. Could be a mix of too much caster, too much rear sway bar, not enough front bar or a few other issues.

I do think it's kind of funny that most of what people do to their cars to make them "handle better" is basically just turning their independent suspensions into more and more like a solid axle. For instance installing super stiff springs or sway bars to try to keep camber from changing through roll.

In a way, that is actually making them the opposite of a solid axle, because bump on one end will result in droop in the other and vice versa, since the springs are by necessity not located over the contact patches. It is sort of like having a built in Z bar.

If it is not apparent, i consider this to be a virtue and not a vice. Especially when judicious amounts of anti squat are present and the goal is to accelerate as hard as practical over rough terrain.

It's kind of funny you mention Z-bars. I'm actually planning on incorporating longitudinal z-bars (probably in the form of lightweight center pivot leaf springs) and having a monoshock/spring at the front and rear. The theory is that it should articulate like a mini rock crawler and keep load variations on the tires to an absolute minimum while driving on uneven pavement.

I think the main advantage for street cars besides unsprung weight (and load floor height which is why even truck based SUVs went IRS) is that if you encounter a bump while loaded up hard in a corner, you are less likely to spin out of control.

I once went a road rally with a car with a stiffened torsion beam that worked well in autocross. On actual hilly curvy (bumpy) roads, i felt it was dangerous to keep pace with IRS cars that were closer to stock. That's anecdotal, but it's the difference between a flat track and a real road.

So if the torque arms are better for weight transfer, and the IRS is better for unsprung weight and not having your shoes tied together... Does the answer become "De-Dion"?

De Dion doesn't give you any anti squat because the drivetrain torque loadings are divorced from the suspension. All a de Deon is, is a really heavy IRS.

I like what BMW did for the E36's ti chassis cars. It's the old trailing arm suspension but they mounted the front on some bushings with a ton of vertical compliance. So you get some anti squat as you accelerate and the nose of the diff pushes the subframe up, altering the geometry for a higher instant center and therefore more anti squat. Not a lot, but some, which is better than none. And then when braking, the brake torque on the trailing arms pushes the subframe down, minimizing the brake hop that you can get with an excessively high/short instant center.

You fix the brake hop problem on solid axle cars by floating the calipers on bearings that have their own linkage to the chassis, so brake torque doesn't go through the rearend the same way...

I think the main advantage for street cars besides unsprung weight (and load floor height which is why even truck based SUVs went IRS) is that if you encounter a bump while loaded up hard in a corner, you are less likely to spin out of control.

I once went a road rally with a car with a stiffened torsion beam that worked well in autocross. On actual hilly curvy roads, i felt it was dangerous to keep pace with IRS cars that were closer to stock. That's anecdotal, but it's the difference between a flat track and a real road.

A well set up solid axle that's not way too heavy for the car it's under isn't too bad about that. Keeping enough sidewall on the tires helps a lot too, as it'll let the tires soak up the small bumps. Having shocks that are well matched to the springs and chassis makes a big difference on this one. Not using too much rear sway bar helps a lot too.

Before building my Locost (and still now, 12 years later), I waffled back and forth between a stick axle and IRS.

My stick-axle Locost has been on the road, travelled 500km on the Trans Canada to autocross events and back, commuted as a daily for a time, and I've driven it everywhere, rain, shine, snow, even dirt roads (-that- sucked).

I've come to a couple conclusions:

1) If you want comfort, you really wouldn't pick a Lotus 7 or replica thereof. Comfort is not their forte.

2) If, on the street, the stick axle is a limited factor, you are likely traveling faster than you legally should be going. If it got you in trouble, it's going to be a big "off."

3) I'm not totally convinced an IRS is going to net a "lighter overall" weight of vehicle, and maximum lightness is what I want.

I don't believe that swapping TO an IRS is going to be worthwhile for -me-. If I was building a project and I had to source an axle, I'd probably go IRS, but the stick works well enough, and if it came with your donor vehicle, or it's already under your project vehicle, it will work fine.

If you want comfort, you really wouldn't pick a Lotus 7 or replica thereof. Comfort is not their forte.

Ha! No kidding. I really, really thought I was tough enough to daily drive a Cobra replica -- but spending the day with one nuked that idea. Comfort is not their forte. My commute involves a 40 mile mix of highway driving at high speeds and bumper-to-bumper traffic, and a Cobra is uncomfortable at both.

Before building my Locost (and still now, 12 years later), I waffled back and forth between a stick axle and IRS.

My stick-axle Locost has been on the road, travelled 500km on the Trans Canada to autocross events and back, commuted as a daily for a time, and I've driven it everywhere, rain, shine, snow, even dirt roads (-that- sucked).

I've come to a couple conclusions:

1) If you want comfort, you really wouldn't pick a Lotus 7 or replica thereof. Comfort is not their forte.

2) If, on the street, the stick axle is a limited factor, you are likely traveling faster than you legally should be going. If it got you in trouble, it's going to be a big "off."

3) I'm not totally convinced an IRS is going to net a "lighter overall" weight of vehicle, and maximum lightness is what I want.

I don't believe that swapping TO an IRS is going to be worthwhile for -me-. If I was building a project and I had to source an axle, I'd probably go IRS, but the stick works well enough, and if it came with your donor vehicle, or it's already under your project vehicle, it will work fine.

"Comfort" is a very subjective term and not something everyone's going to agree on. Your experience with it only being an issue at high speed is probably safety-related, but I want to stick to what's comfortable. I have real-world back-to-back experience with both as well and can say that a heavy rear axle and light car will always make for uncomfortable driving, regardless of speed.

For comparison, my brother has a Seven clone, front LS3 engine and straight rear axle. I have a mid-engine "Seven", with IRS. Both weigh the same, have the same power, and are nearly the same size. We each drove both cars down the same street at about 30 mph. At one point, my brother said I needed to slow down for a bump - a 1"-high step in pavement height. I saw it and didn't slow, and my brother was dumbfounded how he could hardly feel the bump. Later when I drove his car, I understood why he spoke up, as hitting the same bump at the same speed was like driving over a curb - it hurt.

Two things were at work, obviously the straight axle versus IRS, but also sprung-to-unsprung weight. His car is front engine while mine is mid, so my car has a lot more weight over the tires that most affect ride comfort, so the weight ratio is a lot higher. Also, because he's using an LS3, he needs a heavy duty rear axle assembly, and this one weighs 150 lbs just for the axle alone, without brakes, wheels, or tires. As a result, the weight riding directly on the ground is a fairly large fraction of the sprung mass riding on it. The result is that a far greater fraction of a given bump is fed straight through into the chassis - and occupants.

It's interesting that this thread came up now, as just last week we were discussing him converting to IRS. I asked why, because the reason matters. If it's for a faster lap time, I doubt the conversion would help, and might even hurt. On the street though is a completely different story and it absolutely will improve the ride.

So at the end of the day, what's "better" and more "comfortable" are however you define them to be.

I had the same car as Kurt's brother (with a smaller motor). It would happily run with ZO6 Vette's at Sears Point or Laguna Seca, but struggle to keep up with 1st gen Miatas on a poorly maintained twisty road. The difference: purely that rear axle.